The history of electrochemical energy storage devices, especially capacitors and batteries, has involved attempts to reduce package size while increasing the electrical energy storage capacity. Recent advances in battery design have increased life, efficiency and energy density by making improved lead-acid, nickel-cadmium, nickel-zinc and various primary cells. However, although many of the devices embracing the recent technological advances have filled a need, there continues to be a requirement for efficient, high power density electrical storage devices which withstand the rigors of continuous use and virtually unlimited cycling.
Ultracapacitors and supercapacitors are a new breed of energy storage devices that are completely distinctive from batteries. These devices are true capacitors in that energy is stored by the separation of positive and negative charges. However, unlike traditional parallel plate capacitors, these capacitors store charge at the atomic level between the electrode and the electrolyte. This charge storage mechanism is highly efficient and can produce high capacitances up to several hundred Farads in a compact package. These capacitors are available in two basic varieties depending on the composition of the electrodes. Supercapacitors use activated carbon as the electrode element. While this material is inexpensive, the high internal resistance of the activated carbon limits the power available from the storage device. An alternative technology is to use electrodes composed of one or more oxides of ruthenium, tantalum, rhodium, iridium, cobalt, nickel, molybdenum, tungsten or vanadium deposited on a metal foil. Devices made using these materials for electrodes are known as "ultracapacitors" or "pseudocapacitors", and are described in further detail in Canadian Patent 1,270,296, incorporated herein by reference. Although these devices have much lower internal resistance and hence, higher power densities than carbon-based supercapacitors, the materials used are very expensive. Consequently, construction of the precious metal ultracapacitors can cost several hundred dollars each.
Ultracapacitors store energy by two processes which are shown in FIG. 1. The first is the separation of positive and negative charges at the interface between the electrode and the electrolyte. This phenomenon is called double layer capacitance. The electrical double layer is present at virtually all interfaces between electrodes and electrolytes and is a fundamental property of electrochemical systems. The double layer consists of sorbed ions 12 that are specifically sorbed on the electrode 10 from solution as well as solvated ions 14. The proximity between the electrode 10 and solvated ions 14 is limited by the presence of the solvation sheath 16 around the ions, hence, the solvated ions cannot sorb on the electrode and only approach to some distance d. Therefore, in the case of these solvated ions 14, there exists positive and negative charges separated by a distance d (see FIG. 1), which produce a true capacitance in the electrical sense. The second charge storage mechanism is the sorption of ions on the surface of the electrode. This phenomenon is called pseudocapacitance. The key point to note is that pseudocapacitance is not an electrostatic capacitance like that of the double layer or such as occurring in a parallel plate capacitor. Hence, the term "pseudo" (meaning false) capacitance. Instead, pseudocapacitance is merely a convenient formalism used to express the phenomenon of ion adsorption on electrodes, since the electrical response arising from ion sorption mimics an electrical capacitance. Carbon based supercapacitors rely primarily on the double layer capacitance effect for charge storage, while pseudocapacitors rely on both pseudocapacitance and double layer capacitance. As discussed above, both of these processes are surface phenomena and are highly reversible. The physical processes involved in energy storage in a supercapacitor or ultracapacitor are distinctly different from the bulk phase electrochemical oxidation/reduction processes responsible for charge storage in batteries. Hence, these devices represent a class of energy storage materials completely separate from batteries.
The pseudocapacitance in ultracapacitors fabricated from RuO.sub.x and similar metal oxides is due to the following surface reactions which produces a redox couple: EQU 2[Ru(IV)O.sub.2 ]+2H.sup.+ +2e.sup.- .fwdarw.Ru.sub.2 (III)O.sub.3 +H.sub.2 O
Capacitance is generated through the adsorption of protons on the surface, migration of protons into the oxide lattice, and proton and electron hopping throughout the lattice to produce the above redox reactions. It is thought that the interaction effect between the redox couples spreads the reversible current response over a larger potential range than that which might otherwise arise for two separate redox systems.
Clearly, a need exists for an electrical energy storage device that combines the desirable features of precious metal ultracapacitors and conventional electrochemical batteries, yet can be manufactured at a reasonable cost.